Media sheets are moved through an image forming device by a series of rollers and/or belts. In a monochromatic device, the media sheet is moved along a media path past one photoconductive member that forms an image on the sheet with a single toner layer, usually in black toner. In a color device, the media sheet is moved along the media path past a number of photoconductive members that each form a different color toner layer on the sheet. The toner layers are place in an overlapping arrangement and usually include black, yellow, magenta, and cyan toner. The combination of different layers forms a wide spectrum of color images. It is important that the media sheet is accurately moved through the device during the image formation process.
The media sheet should be aligned properly while moving along the media path. A media sheet is aligned if, when crossing a line across the media path perpendicular to the direction of travel, the leading edge encounters the line at the same time along its extent. A media sheet is skewed if, for example, when crossing such a line, one of the leading corners of the media sheet encounters the line before the other leading corner. The toner layers will be placed in a skewed configuration if the media sheet is not properly aligned when moving past the photoconductive members. This results in non-uniform margins along the edges of the printed media sheet.
Another concern for color image forming devices is the media sheet being accurately located while moving past each of the photoconductive members. When the sheet is accurately located, each toner layer is accurately aligned with the other toner layers resulting in a clear color image. If the media sheet is not properly located, one or more of the toner layers will be offset from the other toner layers. This results in a ghosting effect that is of unacceptable quality.
The image forming devices should also be able to produce an acceptable number of printed images per minute. This is important because many consumers base their purchasing decision on the printing speed of the device. Therefore, any methods and devices that prevent media and toner misalignment should not greatly adverse the throughput of the device.